Ocean motion moves microbes to filter light

When simulating ocean currents, scientists
often add particles to the water so they can visualize its movement. Recently, researchers
at MIT used this phenomenon to determine that the microbes found in the ocean also
align themselves with the current and, in so doing, help determine how much light
penetrates the ocean and how much bounces off as backscatter.

Because many microbes, like phytoplankton, have elongated shapes
or live in communities of long chains, scientists have found their orientation to
ocean currents could have a substantial effect on ocean light, which influences
photosynthesis and phytoplankton growth rates. In addition, it could affect satellite
readings of light backscatter used to inform climate models or assess algal blooms.

In a dormant ocean environment, phytoplanktons are oriented randomly,
allowing light to filter easily into the ocean. However, with fluid flow, the random
arrangement changes, affecting the amount of backscatter.

“Even small shear rates can increase backscattering from
blooms of large phytoplankton by more than 30 percent,” said Roman Stocker,
professor of civil and environmental engineering at MIT. “This implies that
fluid flow, which is typically neglected in models of marine optics, may exert an
important control on light propagation, influencing the rates of carbon fixation
and how we estimate these rates via remote sensing.”

In addition to fluid flow, the researchers studied how microbial
size affected light filtration. They observed that very small microbes – less
than 1 μm in diameter – did not align with the ocean current because,
in an effect called Brownian motion, they were too vigorously kicked around by water
molecules. He recreated an ocean environment in microfluidic devices about the size
of a stick of gum and used videomicroscopy to trace and record the microbes’
movement in response to food and current.

The researchers plan to test the mechanism in the field in a local
environment suitable for experimentation, like a nearby lake.

Their findings were published March 8, 2011, in the Proceedings
of the National Academy of Sciences (doi: 10.1073/pnas.1014576108).